patch 8.0.0204: compiler warns for uninitialized variable

Problem:    Compiler warns for uninitialized variable. (Tony Mechelynck)
Solution:   When skipping set "id" to -1.
diff --git a/src/syntax.c b/src/syntax.c
index 68f01c1..e466de8 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -6049,7 +6049,9 @@
 	    }
 	    else if (name[1] == '@')
 	    {
-		if (!skip)
+		if (skip)
+		    id = -1;
+		else
 		    id = syn_check_cluster(name + 2, (int)(end - p - 1));
 	    }
 	    else